The invention relates to a disc brake, in particular for a motor vehicle.
A disc brake comprises two shoes placed on either side of a disc integral in rotation with a wheel of a vehicle, these shoes being clamped on the faces of the disc, in order to develop a braking torque, by hydraulic pressure developed in a braking circuit controlled by the driver acting on a brake pedal.
In general, the shoes are guided by a caliper which is linked by an assembly, sliding along the axis of rotation of the disc, with a fixed yoke. The caliper comprises a blind cylinder in which a piston receiving the hydraulic pressure of the braking circuit is pushed so that it bears on the back of a shoe. The sliding assembly of the caliper makes it possible to distribute the braking forces equally between the two shoes.
An annular seal with a rectangular cross-section is fitted in a groove of the cylinder and comprises a face maintained in pressure against the piston in order to achieve fluid-tightness with respect to the hydraulic fluid acting on the piston.
The width of the groove is greater than that of the seal in order to leave an axial clearance between the seal and the groove to facilitate the assembly and to make it possible to absorb dimensional variations resulting for example from the compression of the seal, manufacturing tolerances or from an expansion of the seal due to heating or to its impregnation by the liquid present.
In a known production, the groove has a cross-sectional shape which widens towards the axis in order to lock as much as possible the outer peripheral part of the seal whilst allowing its inner peripheral part to move axially, following the piston. Thus, for a small movement of the piston produced during a braking, over a travel for example of the order to 0.5 millimeters, the face of the seal in contact with the piston is driven by friction with the piston and the seal undergoes an elastic deformation.
When the driver releases his action on the brake pedal, the seal, regaining its shape, drives the piston which is drawn backwards, which makes it possible to detach the shoes from the disc and thus to prevent residual braking causing wear of the friction pads.
However, the coefficients of friction of the seal on the piston and on the bottom of the groove are difficult to control and the functioning can differ in certain cases. In particular during a retraction of the piston over a larger travel, for example under the effect of a thrust of the disc undergoing lateral forces during a turn, the seal can move axially in its groove and move back with the piston within the limit of the axial clearance.
The functioning of the system is thus disturbed: during the next braking the seal moves with the piston and no longer provides the elastic return function and the sensation at the pedal is modified, which is unpleasant for the driver.